Previously, oil has been extracted from plastic separately collected as a portion of municipal garbage or waste plastic collected as an industrial waste by severing the carbon frameworks at approximately 400.degree. C. to reduce the molecular weight, resulting in liquified oil.
The energy required by this method is approximately 500 to 1,000 kcal/kg (of plastic). To deliver this energy, the liquified plastic is circulated by means of a pump and a furnace is provided on the path of circulation.
In order to prevent thermosetting resins or hard foreign materials from causing trouble in the pump circulation line, these substances must be completely removed through preprocessing. This drives up the cost and creates a problem of economy.
Furthermore, oil obtained solely by pyrolyzing a plastic is unstable. This instability can result in coking on the walls inside the furnace pipes.
In the case of thermoplastic resins, some carbon residue is created in the process of thermal polycondensation, and these particles will be suspended in the pump circulation line. They can be separated from the liquid by passing the plastic through a filter or by centrifuging it, but there is no way to prevent some of the oil from combining with these particles and being lost. The carbon residue also could be a valuable source of energy; however, there is no way to use it efficiently.
Since this liquid oil will solidify at normal temperatures, the pump circulation line must be insulated with a steam jacket. A heavy oil lubricant is required in the processing line, which must be changed each time the line is started up or shut down. This increases the cost of operation.